1. Field of the Invention
The invention relates to a control system and a control method for an electric water pump for use in a cooling system of an internal combustion engine.
2. Description of the Related Art
In a cooling system of an internal combustion engine, a water pump typically circulates a coolant between the internal combustion engine and a radiator. The water pump can be of an internal combustion engine drive type or a motor drive type (electric water pump).
In a water pump of an internal combustion engine drive type, rotation power of a crankshaft of the internal combustion engine is transmitted by a belt to drive the water pump. An electric water pump is typically driven directly by a brushless motor that has a small friction loss.
The related art of cooling systems for internal combustion engines using an electric water pump will be described below.
For example, Japanese Patent Application Publication No. 2002-161748 (JP-A-2002-161748) discloses a cooling system for an internal combustion engine in which drive and stop of an electric water pump are controlled so that a coolant is maintained at a predetermined target temperature.
In this cooling system, when the coolant temperature is equal to or lower than a predetermined value, the brushless motor of the electric water pump is stopped or decelerated, and when the coolant temperature is equal to or above a predetermined value, the brushless motor is driven or accelerated.
In the related art disclosed in JP-A-2002-161748, hunting control is sometimes performed by which drive and stop of the electric water pump are frequently repeated in the vicinity of the target temperature of the coolant, but such a hunting control is easy to perform in a case where the brushless motor of the electric water pump is provided with a sensor for detecting a magnet position of the brushless motor rotor.
However, in recent years, the sensors are sometimes eliminated to make the brushless motors less expensive and more compact. Thus, in a case of a three-phase brushless motor, three magnetic sensors are necessary to know the position (angle) of the rotor for every 60 degrees.
In a case where such a sensorless-type brushless motor is used, when a control system issues a restart instruction in a state in which the rotor rotates by inertia after a stop instruction has been received, as in a mode in which the drive and stop are frequently repeated, the brushless motor synchronism is lost and the rotor is stopped because the rotor magnet position cannot be recognized. As a result, the restart has to be performed after the rotor has been stopped. Therefore, stable regulation of coolant temperature cannot be performed, for example, a time lag from when the stop instruction is outputted till when the restart is actually performed increases. Here, sufficient latitude for improvement opportunity is present.
JP-A-2002-161748 does not describe whether the brushless motor of the electric water pump is provided with a sensor for rotor angle detection, but because no special control is described to be performed when the brushless motor is started and stopped, it can be assumed that this related art requires the use of the sensors.
Japanese Patent Application Publication No. 2000-125584 (JP-A-2000-125584) and Japanese Patent Application Publication No. 2001-113082 (JP-A-2001-113082) (these documents relate to a field different from that of cooling systems for internal combustion engines) describe machines (an air conditioner and a washing machine) using a brushless motor that are provided with a magnetic sensor (Hall sensor) for detecting a position (angle) of the brushless motor rotor. Thus, it can be said that using a sensor for rotor angle detection is typical for control performed to drive and stop a brushless motor.